Summary

Much of Pakistan's 13 million ha of irrigated land is underlain by the Indus River System alluvial complex, which forms a highly transmissive aquifer, often saturated to within a few metres of ground surface. Since the 1960s, public well fields have been installed in this aquifer for water table control and for providing supplementary irrigation water. The early well designs utilized unprotected mild steel components, screens with saw-cut slots and filter packs of ‘pea gravel’. These suffered from a number of drawbacks, including sand pumping and rapid specific capacity deterioration, attributed at the time to corrosion and associated incrustation of the well screens. Design modifications, introducing finer, graded gravel, a more suitable slot size and corrosion resistant materials, resulted in an almost immediate solution of the short-term problems. However, long-term performance monitoring has disclosed slow but serious well deterioration, the causes of which are still not understood. Circumstantial evidence suggests that this deterioration is at least partly due to some chemical mechanism, probably affecting the formation/gravel filter interface. Systematic studies and rehabilitation trials are urgently required to establish effective remedial measures for existing wells and modifications of design for future wells.

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